The present invention relates generally to hydraulic lash adjusters, and more particularly, to a hydraulic lash adjuster (HLA) of the type in which there is both a high pressure chamber and a low pressure (reservoir) chamber.
Hydraulic lash adjusters (also sometimes referred to as “lifters” or “lash compensation devices”) for internal combustion engines have been in use for many years, and serve to eliminate the clearance (or lash) between engine valve train components under varying operating conditions, in order to maintain efficiency and to reduce noise and wear in the valve train. An HLA operates on the principal of transmitting the energy of the valve actuating cam through hydraulic fluid, trapped in a high pressure chamber under a plunger. During each operating cycle of the cam, as the length of the valve actuating components varies, as a result of temperature changes and wear, small quantities of hydraulic fluid are permitted to enter the pressure chamber, or escape therefrom, thus effecting an adjustment of the position of the plunger, and consequently adjusting the effective total length of the valve train.
The typical, prior art HLA comprises a generally cylindrical, cup-shaped body member which is disposed within a cylindrical bore defined by the engine cylinder head. Disposed within the body is a plunger assembly which is slidingly received within a blind bore defined by the body member. The lower end of the plunger assembly cooperates with the blind bore to define the high pressure chamber. In the conventional HLA, when a load is applied to the plunger assembly (from the cam profile, by means of a rocker arm), the load increases the pressure of the hydraulic fluid within the high pressure chamber, and fluid escapes the high pressure chamber through a cylindrical clearance defined between the blind bore and the outer cylindrical surface of the plunger. An HLA of the type described is referred to as a “conventional leakdown” lash adjuster. Although the present invention could be utilized in conjunction with various other types of HLA, it is especially adapted for use in an HLA of the conventional leak-down type, and will be described in connection therewith.
In a conventional leakdown HLA, in which the leakdown clearance is defined between the body bore and the plunger outer surface, it is understood by those skilled in the art that the diameter of the blind bore defined by the body must be maintained within a very tight tolerance range. Typically, the final step in machining/sizing the body bore is a grinding operation which, as is well known to those skilled in the art, tends to be a fairly expensive operation, in part because of the cup-shape of the body. Even after such an expensive grinding operation on the body bore, it is typical in the HLA art that the bodies and plungers are “sized and sorted” with regard to the body bore inner diameter and the plunger outer diameter, in order that, after assembly, each body-and-plunger pair has a leakdown clearance within the desire tolerance range. Even after the size and sort operation, and the match fitting of the plunger and body, it is fairly common to have leakdown performance outside of the tolerance range. When such unacceptable performance is identified, subsequent to assembly, it is then necessary to dis-assemble the HLA, and re-assemble the body and plunger from that HLA with other components, in an attempt to achieve HLA performance within the tolerance range. All of that type of “re-work” is time consuming and expensive, and should be avoided to the extent possible.
Those skilled in the art of hydraulic lash adjusters understand that, even though the rocker arm imposes a generally axial load on the plunger of an HLA, there is typically also a side load component applied to the plunger. As is also now well known, any such side load imposed on the plunger will effectively change the leakdown clearance between the body and the plunger, thus resulting in undesirable variations in the leakdown performance of the HLA.